1. Field of the Invention
The present invention relates to a signal circuit for use in an automatic focusing lens which can be coupled to a camera having an automatic focusing system capable of automatic focus detection, and which moves the lens in accordance with an output from the automatic focusing system.
2. Description of the Prior Art
There are conventionally, a camera capable of an automatic focus detection, and an automatic focusing lens having a lens driving system for driving a lens on the basis of the detection information from the camera when the automatic focusing lens is connected to the camera. It is also well known, for example, in U.S. Pat. No. 4,067,031 (issued on Jan. 3, 1978) that in the automatic focusing lens, the lens is moved by a motor and that there is provided a limit switch which is changed-over when the lens has been moved to the infinity or minimum-focusing-point so that power supply to the motor is inhibited, and that when the lens has been moved to the infinity or minimum-focusing-point, the succeeding movement of the lens is inhibited.
On the other hand, in Japanese Patent Application Nos. 204953/1981 (filing date of the application is Dec. 17, 1981) and 205786/1981 (filing date of application is Dec. 18, 1981), owned in common herewith, the present applicants have already proposed a signal circuit for an automatic focusing lens having a displacement monitoring signal generator for generating a pulse-like displacement monitoring signal which represents the displacement of the lens by repeatedly turning on and off in accordance with the movement of the lens in order to control the displacement of the lens by a motor; wherein by detecting that the displacement monitoring signal to be output in response to the power supply to the motor is not provided for a predetermined period of time, the above-mentioned signal circuit discriminates whether or not the lens has been moved to the infinity or minimum-focusing-point, so that the circuit carries out the necessary operation such as stoppage of the power supply to the lens driving motor or the like. This automatic focusing lens has the advantage that it is possible to easily discriminate whether or not the lens has moved to the infinity or minimum-focusing-point in accordance with the detection of the displacement monitoring signal necessary to control the displacement of the lens; however on the contrary, it has a disadvantage, as follows. That is to say, for example, in the case that the generator for generating a pulse-like displacement monitoring signal in accordance with the movement of the lens is constituted by a comb tooth-like contact member and a brush adapted to slide on this comb tooth-like contact member in association with the movement of the lens, if the lens locates at the infinity or minimum-focusing-point and the brush is positioned at the change point of the comb tooth-like contact member, a pulse-like signal is generated from the output terminal of the displacement monitoring signal due to chattering, so that the discrimination that the lens has moved to the limit position cannot be performed.